It is often necessary to divert a stream from a section of its natural streambed during various construction projects, such as road, bridge, or buried pipeline crossing construction. The stream diversion is required to avoid disrupting the stream flow and releasing turbidity into the downstream waters. A typical construction activity that requires this “isolate and bypass” method is the installation of a buried pipeline crossing under the streambed. In order to minimize the environmental disturbance to aquatic life in the stream, the natural water flows must be maintained from one side of the project to the other during construction, and sediments from the construction operation must not be allowed to mix with the flow. It may also be necessary to allow “dry” access to the diverted section of streambed for excavation or other construction activities.
The normal “isolate and bypass” construction practice is to install upstream and downstream cofferdams and divert the flow through some form of conduit, commonly a channel, hose, flume or duct. The flow diversion may be achieved by excavating a bypass flow channel adjacent the natural streambed, installing pumps and pipelines, installing flumes, or installing large box shaped flow channels commonly known as “superflumes” if the natural volume of flow is substantial. Upon completion of construction, the streambed is appropriately restored and flow is returned to the streambed following removal of the temporary flow bypass system.
The diversion, or bypass system must have the capacity to handle the range of stream flows experienced while it is in use. In many streams flow will be nominal. However a very high potential stream surge flow, or “freshet”, may be experienced after a precipitation event. If the diversion system cannot handle such flow volumes, a catastrophic flooding of the construction site may occur, with significant downstream environmental impacts. Statistical analysis of the historical stream flow rates during the planned season of construction activity is required to estimate the likely and potential maximum flows to be diverted.
In situations where the stream flow rates are expected to be significant, exceeding the typical capability of pumping systems, long culverts, or “flumes”, made of large diameter steel pipe are often used for the diversion. The flume spans between the upstream and downstream cofferdams in a straight line, and is of sufficient diameter to handle the excess flow.
The flow capacity of the flume is limited by the hydraulic head difference between the water levels behind the cofferdams. The upstream dam must back up the stream sufficiently to produce adequate hydraulic head differential to induce the required flow in the flume such that a normal volume of flow is delivered past the diverted section of streambed. In some cases both the upstream and downstream water level is above the flume elevation and it operates as a simple flowing water pipe where the flow is dependent on the difference between the upstream and downstream heads. In other cases the downstream water level is lower than the flume discharge and the flume operates as a partially filled culvert where the flow rate is governed by difference between upstream head and the flume elevation. This hydraulic condition is known as “inlet controlled”, as the flow is limited by the upstream head and its inherent ability to flow water into the entrance of the flume.
In streams where very large flow volumes must be diverted, the required capacity cannot be provided economically with the large pipe flumes discussed above. On these rare occasions, larger box shaped flumes, often called “superflumes”, are used to provide a larger conduit cross sectional area to accommodate larger anticipated flows. These structures are necessarily huge, heavy, and expensive to build. They are also expensive to transport and install. Another disadvantage to the large size of these superflumes is the resulting limitation of access to the streambed for excavation and underground pipe installation.
Further, the flow in a box channel “superflume” is generally limited by inlet conditions. To achieve high flow rates it is necessary to dam the upstream water level to significant depths. Since the total force on the dam increases approximately with the inlet depth squared, this becomes an expensive undertaking as well.
The high discharge flow velocity out of flumes generally poses an erosion problem in the downstream bed. Thus, the downstream flow is often manually altered upon exit from the flume by placement of rocks or rubble within the streambed to increase turbulence and thereby diffuse discharge flow from the flume.
In larger streams, the total construction process to install and remove a large superflume system may substantially increase the duration of the in-stream construction activities. This increases the environmental impacts to the stream and also increases the probability that a stream freshet will occur during the isolate and bypass operation.
Flexible tubes have been used to divert water from a stream. U.S. Pat. No. 5,242,244 to Dockery describes a sleeve for isolating a flowing water stream from surrounding stagnant waters so that selective chemical treatment of the stagnant water is possible.
U.S. Pat. No. 5,947,640 to Connors describes a flexible tube system for conveying water past a construction site in a streambed. This invention provides an economical and highly portable method of conveying water through a flexible tube that must be continuously supported along its length. No significant measures to enhance the hydraulic capacity of the tube are considered and the flexible wall tube must operate at or above atmospheric pressure to avoid collapse.
U.S. Pat. No. 1,984,802 to Mallery discloses another flexible tube system for conveying water between two points in a stream so that the “dry” streambed can be accessed for mining and other operations.
Numerous water conveyance systems have been developed to facilitate the passage of fish around stream obstructions such as hydroelectric dams. These systems typically operate by maintaining a flow of water through a conduit having internal passageways or resting places for fish, to aid in passage over a dam or past a site of stream disruption. Flow through the passageway is maintained or supplemented by pumps.
In recent years the level of environmental protection required during construction activities has increased significantly, particularly in the pipeline construction industry. Large pipeline projects, involving the crossing of hundreds of streams, are restricted to using “isolate and bypass” construction methods. More efficient high capacity stream diversion systems are needed.
A key technical objective in providing an improved water bypass system is the efficient achievement of high flow velocity when necessary, while minimizing conduit size and weight. Further, reliable operation at varying flow rates with minimal hydraulic head (provided by the stream and cofferdam systems) is desirable.